Understanding the two main classes of pumps in engineering: positive displacement and centrifugal

Two big families, two different jobs: in engineering, pumps split neatly into positive displacement and non-positive displacement. If you’re studying for the BDOC’s engineering topics, this distinction is a reliable compass for a lot of design and maintenance questions. Let me walk you through what each class does, where they shine, and how you’d pick one for a real world task.

Positive displacement: a measured bite of fluid every cycle

Think of a positive displacement pump as a precise feeder. It traps a fixed amount of fluid, then pushes it out through the outlet. Because it’s delivering a set chunk with each cycle, the flow rate stays predictable even if the system pressure changes. That’s why these pumps are the go-to when you need a steady, high-pressure output, regardless of how the surroundings are behaving.

What kinds fit here? Gear pumps, diaphragm pumps, and piston pumps are classic examples. Each one traps a pocket of liquid and then “milks” it forward. The details differ—gears meshing to snag a slug of fluid, diaphragms flexing to squeeze a chamber, pistons pushing against a known volume—but the core idea remains the same: you control how much liquid moves per revolution or stroke.

Where they shine

• Consistent flow against higher pressures: great for dosing, metering, or moving viscous fluids that don’t want to cooperate with gravity.

• Tolerant of sudden pressure changes: because they don’t rely on speed alone to push fluid, they still deliver a known amount each cycle.

• Good for tricky fluids: thick oils, slurries, or sludges that would stall a gentler pump.

Where you’ll want to be careful

• Pulsating flow: the output can come in bursts, which isn’t ideal for every system. You might tame this with dampeners or accumulator hardware.

• Prime and seal needs: some designs need careful priming and robust seals, especially if you’re pulling liquid from a low point or handling gases.

• Maintenance touchpoints: moving parts wear out. Bearings, seals, and diaphragms demand regular checks.

Non-positive displacement: the fluid river, not a fixed bite

Non-positive displacement pumps—often called centrifugal pumps—work by spinning an impeller to fling fluid outward, creating velocity. The kinetic energy then translates into pressure as the liquid slows down inside the volute or diffuser. The key here is continuous flow: these pumps don’t trap fixed volumes the way positive displacement units do. They excel when you’re dealing with large volumes of fluid and you can tolerate a bit of variability in pressure at lower end of the flow range.

Centrifugal pumps is the common name you’ll hear, and they come in all sorts of sizes and materials. They’re the workhorses for irrigation, building cooling loops, water treatment, and the like. They’re also part of many power plants and industrial plants where you need to move a lot of liquid efficiently.

Where they shine

• High flow, lower to moderate pressure: great when you need to move a lot of liquid quickly.

• Smooth, continuous output: fewer pulsations mean simpler downstream control and measurement.

• Economy and simplicity: generally straightforward to install and maintain, with fewer moving parts than some positive displacement designs.

Where you’ll want to be mindful

• Sensitivity to system conditions: if the discharge pressure rises quickly or the flow path is highly restrictive, the pump can stall or lose efficiency.

• Priming matters: many centrifugal pumps need to be primed and must be filled with liquid before start; air pockets can cause loss of prime and cavitation.

• Not ideal for high-viscosity fluids: thick liquids slow the impeller and reduce performance, sometimes to a crawl.

• Net positive suction head (NPSH) concerns: you’ve got to make sure the pump can avoid cavitation, especially in cold or viscous systems.

A quick side-by-side view, so you can picture it

• Flow behavior: positive displacement pumps deliver a fixed amount per cycle; centrifugal pumps deliver flow that scales with speed and system pressure.

• Response to pressure: PD pumps keep pumping at a set rate; centrifugal pumps creep up or down with the system head.

• Fluid types: PD pumps handle thick or particulate-laden fluids better; centrifugal pumps are great for clean, lower-viscosity liquids in big volumes.

• Maintenance vibe: PD often wears seals and moving parts; centrifugal can be a bit more forgiving but needs attention to cavitation and bearing wear.

Why this distinction matters in real engineering work

In the field, you won’t pick a pump by looks alone. You need a sense of the job at hand: desired flow rate, required pressure, fluid type, and how the rest of the system behaves. Here are a few practical touchpoints many BDOC modules touch on:

• Material compatibility and fluid traits: If you’re moving dünne water-like fluids, a centrifugal pump often makes sense. For thick oils, slurries, or precise dosing, a positive displacement pump might be a better fit.

• System curve reality check: Pressure versus flow isn’t a straight line in many systems. Positive displacement pumps can push high pressure even when flow is modest, while centrifugal pumps want to see enough flow to stay happy and efficient.

• Efficiency and cost of ownership: PD pumps can be more mechanically complex, which means maintenance discipline matters. Centrifugal pumps are generally cheaper to run at large volumes but require attention to priming, cavitation risks, and bearings.

• Startup and priming: Some applications demand pumps that start instantly and stay steady; others tolerate a bit of lag as the system fills. The choice can save you headaches later.

• Viscosity and particulates: Fluid science isn’t glamorous to think about, but it’s decisive. Thick fluids or solids-heavy slurries often lean toward PD options; clean, low-viscosity liquids can be a natural fit for centrifugal workhorse setups.

A few real-world anchors you’ll recognize

• Water treatment plants regularly juggle large flows with centrifugal pumps for circulating and delivering treated water.

• In manufacturing lines, dosing pumps (a kind of positive displacement pump) ensure exact quantities of additives or lubricants make it where they’re needed.

• Fuel transfer and lubrication systems often rely on a mix: centrifugal pumps for volume in steady streams, PD pumps when you need a precise, high-pressure slug of fluid.

A simple way to remember it

If you imagine pumping as feeding a pipe, the PD family is like well-timed scoops—each scoop is the same size, and you can push as hard as your system allows. The centrifugal family is more like a river: faster flow as you push the valve wide, slower if you choke the path. Both are useful, but the choice steers the whole project’s behavior.

A quick mental checklist for selecting

• Do I need a precise amount per beat? If yes, lean toward positive displacement.

• Will the system tolerate variable flow and high volume? Then centrifugal might be your friend.

• Is the liquid thick or contains particulates? PD pumps often handle that better.

• Is space or initial cost a concern? Centrifugal pumps tend to be simpler and cheaper upfront.

• Do I worry about priming or cavitation? Factor those in early.

Bringing it all together

The BDOC landscape rewards clarity on fundamentals. Recognizing the two pump families—positive displacement and non-positive displacement—gives you a sturdy framework for diagnosing flow problems, designing a system, or conducting a thoughtful equipment audit. It’s less about memorizing every detail and more about matching the pump behavior to the job that needs doing. When you’re faced with a piping diagram or a spec sheet, that one-line distinction acts like a map.

A closing thought

Pumps aren’t just devices; they’re little engines of reliability in a larger system. Understanding their core operation helps you predict how a system will respond to changes in pressure, viscosity, or demand. So, next time you see a pump lineup, you’ll naturally ask: Is this a fixed-volume workhorse or a high-volume mover? The answer will point you toward the best fit with confidence.

If you want to keep the conversation going, think of a project you’ve encountered—perhaps a cooling loop, a chemical dosing line, or a water supply feed. Try labeling each candidate pump with the two big families and jot down what that means for flow, pressure, and maintenance. You’ll probably notice the little dance between performance, cost, and reliability taking shape right in front of you. And that’s when the theory clicks into something you can act on with clarity.